Furnace circuit for variable pressure once-through generator

ABSTRACT

A variable pressure vapor generator of the once-through type in which circulation is established at normal loads by the generator feed pump. However, at start-up and loads below about 50 percent, evaporation in the generator furnace enclosure is carried out only to a vapor-liquid mixture. At least a minimum flow, for instance about 10 percent flow, is maintained by the generator feed pump, and circulation in the furnace enclosure is established by natural recirculation of the liquid portion of the vapor-liquid mixture, assisted by the feed pump flow. By the invention, turbine throttle pressure is varied in response to load demand, and fluid pressure within the vapor generator circuit is varied essentially in step with the throttle pressure.

Gorzegno 1451 Feb. 5, 1974 FURNACE CIRCUIT FOR VARIABLE PRESSURE ONCE-THROUGH GENERATOR [75] Inventor: Walter P. Gorzegno, Florham Park,

[73] Assignee: Foster Wheeler Corporation,

Livingston, NJ.

[22] Filed: Dec. 27, 1971 [21] Appl. No.: 211,899

52 us. c1...... 122/406 R, 122/406 s, 122/406 ST 51 Int. c1... E22d 7 00 [58 ETeTd of Search 1 22/406 R, 406 S, 406 ST,

Primary Examinerl(enneth W. Sprague Attorney, Agent, or Firm-Marvin A. Naigur; John E. Wilson [5 7] ABSTRACT A variable pressure vapor generator of the oncethrough type in which circulation is established at normal loads by the generator feed pump. However, at start-up and loads below about 50 percent, evaporation in the generator furnace enclosure is carried out only to a vapor-liquid mixture. At least a minimum flow, for instance about 10 percent flow, is maintained by the generator feed pump, and circulation in the furnace enclosure is established by natural recirculation of the liquid portion of the vapor-liquid mixture, assisted by the feed pump flow. By the invention, turbine throttle pressure is varied in response to load demand, and fluid pressure within the vapor generator circuit is varied essentially in step with the throttle pressure.

28 Claims, 14 Drawing Figures SU PHTR. T

BURNER ZONE , PARTIAL 01v. WALLS 4 81 ROOF HRA SPRAY PROGRAM MW VS P DEAERATOR 1 H P. HTRS DEAERATOR LOW PRESS. (IF USED) STORAGE HTRS.

{ FEED PUMP 10% MIN PATENTED 3789.806

SHEET 1 0? I 4000 SUPER HEATER 0UTLET\\ FuRNAcE-- o0 SUPERCRITICAL 3 S 3000 SUBGRITIGAL 2 25ool w TURBINE A 2000* THROTTLE 3 I500 LLI '35 I000? 500 TURBINE STOP VALVE BYPASS O I I l I I I I l I I 0 I0 20 so so 8O I00 LOAD l6 I8 20 52 r PARTIAL DIV. WALLS PRIMARY V FIN. 64 a ROOF HRA SUPHTR. T SUPHTR. so

LOAD 58 SPRAY PROGRAM MW vs P HP. 22

I 7 26 (0ND 28 BURNER zoNE so I DEAERATOR LOW PRESS. A (IF USED) STORAGE HTRS. l T I 5.25. i I 36 34 FEED L. PUMP [NE/HWITW WALTER GORZEG/VO PATENTED FEB 51974 SHEET 3 0F 7 INVENTOR.

ALTER GORZEGNO a I flL PAIENTEDFEB 519M 3.789.805

SHEET 5 0F 7 LINE 401 FINISHING SUPERHEATER PRIMARY SUPE HEATER ROOF PASS 5 HRA PARTIAL I PASS 4 FURNACE I32 DIVISION WALLS II UPPER SIDEWALL I S 5 (CENTER PORTION) SPRAY FROM ECON. INLET FRONT,REAR AND SIDE WALL END PANELS \-I30 PASS 2 WATER COOLED PASS I LOWER SIDEWALL \-I|6 (CENTER PORTION) ECONOMIZER -Il4 v INVENTOR. WALTER I? GORZEG/VO PATENTED 7 sums or 7 0 m E v P. Q fi W a x] U C w Nfi I F: 5E \NW J g 1 mill: m mm: M m WI w ME? 6 NW if H H FURNACE CIRCUIT FOR VARIABLE PRESSURE ONCE-THROUGH GENERATOR BACKGROUND OF THE INVENTION The present invention relates to once-through vapor generators, and in particular to a method of operating and apparatus for once-through generators wherein a jet-pump assisted natural circulation flow is employed for start-up and low loads and the fluid pressure within the generator circuitry varies with load demand.

The present invention is particularly applicable to once-through vapor generators capable of supercritical operation at full load, and will be described with reference thereto, although it will be appreciated that the invention has broader application.

For purposes of the present application, a oncethrough vapor generator is defined as one in which the vapor generation rate is equal to the total fluid circulation rate, and forced circulation at normal loads is obtained by the generator feed pump, the fluid passing progressively through preheating, furnace and superheating sections. The furnace section or enclosure is sized so that at normal loads evaporation therein proceeds to a. high quality mixture or complete dryness.

Prior US. Pat. No. 3,504,655, assigned to assignee of the present application, describes the basic concept of a vapor generator capable of operating by natural circulation during startup and by forced or once-through circulation during normal high pressure operation. However, in the particular mode of operation of the generator which was described in the patent, natural circulation is maintained to about 30 percent load and a pressure of about 2500 p.s.i. At that point, the difference between the density of the mixture in the furnace enclosure risers and the density of the mixture in the downcomers becomes too small, with too little available force or head, to promote natural circulation. Accordingly, the furnace enclosure and other circuits of the vapor generator are then pressurized from 2500 p.s.i. to the full pressure level set by design, for instance 3500 p.s.i. for a supercritical unit. A once-through circulation is thus established with the flow and firing rate varied proportional to load demand.

The trend in the generation of electrical power, with increased availability of nuclear power, is in the direction of more frequent use of large fossil-fired vapor generators to maintain the cyclic or peaking load requirements of a utilitys distribution system. Such use requires that even the more efficient fossil-fired vapor generators be capable of rapid load changes on line for frequency stabilization, and also be capable of rapid warm starts following overnight or weekend shutdowns. For shift-to-shift operation, a distribution system today may require as much as 3 percent per minute load change capability in a high efficiency type (nonpeaking) fossil-fired vapor generator. It is predicted that the load change capability required in future years will increase to more than 5 percent per minute, and in some instances, a load change rate as high as percent per minute may be required. These load change rates do not refer to infrequent operation but to continued and frequent operation where the equipment must sustain many cycles.

Rapid load changes from the point of view of turbine life can best be accomplished by varying throttle pressure at full-arc turbine admission. Again we refer to the high efficiency turbine of large size.

For the large turbine-generator, good operating practice demands that fluid temperature changes throughout the stages of the machine be limited in magnitude and rate of change; to keep resultant induced thermal stresses at minimal values so that cyclic life of the machine, in particular the large rotor mass, is extended and fatigue damage minimized. Turbine rotor damage reduces turbine fatigue capacity and is cumulative. This cumulative damage is a function of how many times each type of operation is performed. Rapid load changes accomplished where initial and final throttle vapor conditions yield large'changes in first stage temperature will cause proportionally greater rotor damage. Variable throttle pressure operation with full-arc admission for the turbine produces minimal change to first stage temperature, and therefore can accommodate rapid load changes with minimum rotor damage.

ln addition during cold and hot starts for the turbine generator, close matching of fluid temperature to turbine metal temperatures is more easily achieved at variable pressure and is desired to limit fatigue damage to inlet parts and shell portions adjacent to the first stage outlet.

Another advantage of close matching of fluid temperature to metal temperature throughout the machine, during start-up and load change operation, is avoidance of excessive vibration and/or rubbing of the machine.

SUMMARY OF THE INVENTION Despite the above advantages, a once-through vapor generator designed for true variable pressure operation is given to more difficult operating conditions, particularly with respect to maintaining satisfactory characteristics of flow in the furnace circuitry, and keeping flow changes within individual tubes caused by heat upsets to these tubes within design limits.

A furnace circuit design capable of assuring functionally satisfactory performance at one load (and pressure) may not, because of changes in the quality of the flow with load and pressure, be capable of functionally satisfactory performance at another load and pressure.

start-ups.

Another object of the present invention is to provide an improved vapor generator the capabilities of which more closely fit the needs of present-day turbines, and particularly a vapor generator of once-through design capable of true variable pressure operation with fullarc admission for the turbine.

It is also an object of the present invention to provide an improved once-through vapor generator in which operation during start-up and partial loads is at least in part by natural circulation wherein the transition from recirculation to once-through is inherently automatic within the circuitry.

To the accomplishment of the foregoing and related ends, the-invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but several of the various ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS The above brief description as well as further objects, features, and advantages of the present invention will be more fully appreciated by referring to.the following description of a presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in connection with the accompanying drawings, wherein:

FIG. 1 is a graph illustrating the relationship of throttle pressure versus load for a vapor generator in accordance with the concepts of the present invention;

FIG. 2 is a schematic diagram of a fiow circuitry for a once-through vapor generator illustrating an embodi ment of the present invention;

FIG. 3 is an elevation section side view of a vapor generator employing the flow circuitry of FIG. 2;

FIG. 4 is a section elevation front view of the vapor generator of FIG. 3;

FIG. 5 is a detailed illustration of a furnace crossover circuit for use in the flow circuitry of the generator of FIG. 3;

FIG. 5A is a section view taken along line 5A5A of FIG. 5',

FIG. 5B is a section view taken along line 5B5B of FIG. 5A;

FIG. 6 is a plan section view ofa vapor-liquid separator suitable for use in the flow circuitry of FIG. 2;

FIG. 6A is an elevation section view of the separator of FIG. 6 taken along line 6A6A;

FIG. 7 is a graph showing load versus circuit flow for a generator in accordance with the concepts of the present invention;

FIG. 8 is a section elevation view ofajet-pump to be employed in the circuitry of FIG. 2 in accordance with the concepts of the present invention;

FIG. 9 is a block flow diagram of a flow circuitry illustrating another embodiment of the present invention; and

FIGS. 10 and 11 are side and front elevation section views ofa once-through vapor generator employing the flow circuitry of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, and in particular to FIG. I, the operation of the vapor generator of the present invention is such that the turbine throttle pressure is increased in response to load demand. A minimum vapor generator circuitry pressure is held to approximately 1000 p.s.i. In the range between 25 percent load and 100 percent load, the fluid pressure within the vapor generator circuitry varies essentially in step with the throttle pressure. Below about 25 percent load, the flow to the turbine is throttled through a turbine stop valve bypass. The transition from subcritical to supercritical flow in the generator circuitry in ramping from 1000 p.s.i. to 3500 p.s.i. (25 percent load to 100 percent load) occurs at 3206 p.s.i. It should be understood that the above values for load vs. pressure are typical only, and may vary depending upon the specific design of the vapor generator. The significant feature is that the vapor generator employs a true variable pressure operation. In addition, the pressures in the furnace circuitry are substantially the same as those in the other pressure parts of the generator, allowing for normal pressure losses, and no pressure breakdown is employed between the furnace circuitry and such other pressure parts.

With reference to FIG. 2, the vapor generator comprises a main flow path 12 which includes the furnace circuitry 14, partial division walls and roof tubes 16, a primary superheater 18, and a finishing superheater 20, the outlet of which is connected to high pressure turbine 22. From the high pressure turbine, the flow is to a reheater 24, a low pressure turbine 26, condenser 28, low pressure heaters 30, and deaerator 32. The flow is then transmitted by means of feed pump 34 to high pressure heaters 36 (if used) and from there to economizer 38 and line 40 into downcomer 42. The latter transmits the flow back to the furnace circuitry completing the once-through cycle.

In accordance with the present invention, the generator further includes a bypass line 44 which leads from a collecting header 48 back to the deaerator 32, or alternatively to high pressure heaters 36. A plurality of separators 50 are disposed at the top of the generator furnace connected to the furnace circuitry 14 by risers 58. A line 52 connects the top outlets of the separators to the partial division walls 16, and a second line 54 connects the lower outlets of the separators to the abovementioned collecting header 48.

Further details of the separators are illustrated in FIGS. 6 and 6A. Essentially the separators are of a known central arm design for which tests and operating experience are available. The separators comprise an outer upright cylindrical shell 56. An inlet tube 58 leading from the furnace circuitry headers extends upwardly coaxially within the shell. The inlet tube is closed at its upper end 60, except for the provision of radially extending curved arms 62 arranged to transmit the flow from the inlet tube tangentially against the inner wall of the shell 56. Separation of the flow (when it is a vapor-liquid mixture) into separate liquid and vapor streams occurs within the upright shell 56. A vapor connection 64 (the top outlet of the separators) passes thevapor stream to line 52 and a liquid connection 66 (the lower outlet) passes the liquid stream to the line 54 and collecting header 48. The liquid (or drain) connection 66 is at the elevation at which it is desired to hold liquid level. In the present example, the drain connection is a 4 /2 inch O.D. line.

In place of the separators, it would be possible to employ a conventional boiler drum of perhaps 60 inches in diameter. However, such a drum would be difficult to design for supercritical pressures (3600 p.s.i.) whereas the individual separators of approximately 11 inches in inside diameter are easily capable of withstanding supercritical steam pressures without requiring excessive wall thicknesses.

The collecting header 48 is connected to the downcomer 42, by line 68, in addition to bypass line 44. Accordingly, the separators 50, lines 54 and 68, collecting header 48 and downcomer 42 establish, with the furnace circuitry 14, a natural circulation loop by which circulation in the furnace circuitry can occur. By means of this loop, a circulation driving force is established due to the difference between the density of the vapor and liquid mixture in the furnace circuitry risers and the density of the saturated liquid in the downcomer 42.

It is a feature of the present invention that the circulation rate in the furnace circuitry is assisted by means of the energy in the flow from the feed pump 34. In the embodiment illustrated in FIG. 2, the downcomer 42 comprises at its upper end a curved elbow 70, which connected to downcomer with line 68, and a jet pump 72, the details of which are illustrated in FIG. 8. A nozzle 74, connected to line 40 leading from the economizer 38, protrudes into the downwardly extending portion of the elbow 70. Below the elbow, the jet pump comprises a venturi 76 having an inlet 78, a throat section 80 and a diffuser 82. The flow from the nozzle constitutes a driving force inducing a secondary flow through the elbow 70. Both flows mix in the inlet 78 and throat section 80 of the venturi. The diffuser section 82 converts the velocity pressure for the mixture to static pressure. The result is a net increase in pressure, through momentum exchange, for the driven secondary fluid at the water jet pump outlet, or in downcomer 42.

Also constituting part of the feed system to the furnace is a feed line 84, containing a pressure reducing control valve 86. This feed line is connected directly between line 40 from the economizer 38 and the downcomer 42 (at a point downstream of the venturi 76). The control valve 86 in the feed lines maintains a pressure ratio (Pg/P2) of about 1.33 between the pressure in the jet pump nozzle 74 and pressure in the venturi inlet 78. This pressure ratio insures proper operation of the jet pump.

A second control valve 88 is positioned in the bypass line 44. This valve controls the level of the liquid in the separators 50, opening and closing in response to the level within the separators. During operation of the generator at loads where circulation is assisted by feed pump flow through the jet pump 72, it is possible to introduce into the furnace circuitry a flow which exceeds I vapor production from the circuitry. Flooding of the separators 50 could occur. A low water level does not impair the functioning of the separators, but a high water level causing flooding would impair such functioning. Accordingly, excess flow in the furnace circuitry is transmitted through valve 88 in bypass line 44 I back to the deaerator 46 or high pressure heater 36 (if 90 (FIG. 2) positioned in the curved elbow at the upper end of downcomer 42. Operation of the check valve is automatic, the valve closing when the feed pressure in the downcomer 42 exceeds jet pump capability. In other words, as pressure and load in the generator is increased, the total pressure drop of the flowing mixture in the furnace enclosure exceeds the net driving head available in the natural circulation loop. Even with jet pump circulation assist, a point is reached at which there is inadequate pressure differential available. At that point, the check valve automatically closes and the flow proceeds in a once-through mode through the generator.

It is a feature of the present invention that at the point of conversion of once-through operation, evaporation in the furnace circuitry 14 is carried out at least essentially to dryness. In other words, the flow into the separators is in a vapor state or a high quality mixture at worst; and liquid recycle if required would only be to a small extent.

By way of example, the generator may initially be started with the 10 percent minimum flow (mentioned above) through the feed pump 34. This minimum flow is transmitted from the economizer through the furnace circuitry and into collecting header 48. Valve 88 controlling level in separators 50 and check valve 90, cause the flow to be recirculated in bypass 44 to either the deaerator storage tank 46 or the high pressure heaters 36 (if used). The pressure within the circuitry at this point may be about 700 p.s.i. The burners are then fired and the recirculation rate in the furnace circuitry is established at about 45 percent of full load flow, by natural circulation with jet pump assist provided by the 10 percent minimum tlow from the feed pump. A load is applied to the turbine at the point when sufficient vapor is being generated and pressure in the generator is-increased to and held at about 1000 p.s.i. Between about 10 percent load and about 25 percent load, the furnace circuit flow is maintained relatively constant (about 45 percent) by jet-pump and adjustment of the feed pump flow. Throttle pressure upstream of the turbine stop valve is maintained by firing rate adjustment, and load control is accomplished by throttling through a turbine stop valve bypass. Feed pump flow in excess of the vapor flow which is generated is recirculated by means of bypass 44.

From about 25 percent load to about 45 percent load, furnace circuit flow is still maintained at about 45 percent of full load flow, but generator pressure is increased by firing rate adjustment from about 1000 p.s.i. to about 2000 p.s.i. in proportion to load. In this period, feed pump flow will also be increased from about 25 percent to about 45 percent of full load flow to maintain furnace circulation, and level in separators 50.

At about the 45 percent load point, check valve 90 automatically closes and conversion to once-through operation occurs. Subsequently, at about 3206 p.s.i.

is maintained up to about 45 percent of full load flow.

Natural or jet-assisted circulation is maintained in the generator only up to this predetermined load point, after which the generator becomes once-through, and continues in that mode up to percent load. This conversion is accomplished by means of a check valve (72% load) flow in the generator converts fr0m-subc rit ical to supercritical. In the range between about 2000 p.s.i. and full pressure (3500 p.s.i.) turbine throttle pressure and steam generator circuitry pressure still increase proportionately, by adjustment of pumping and firing rate.

FIGS. 3 and 4 are sectional side and front views of the peaking type vapor generator of FIG. 2. Preferably,

the furnace flow circuits are of the once-up-through design, that is, having continuous tube routings from the inlet headers to the outlet headers without intervening headers. An advantage of such routing is that, with variable pressure operation, flow of vapor and liquid mixtures entering a circuit which is in a high heat zone is avoided.

However, the furnace circuitry design must exhibit satisfactory characteristics of flow. That is, flow changes within individual tubes caused by heat upsets to these tubes must be kept within design limits.

A satisfactory characteristic of flow can be obtained in part by installing inlet orifices or similar resistance devices (items 92 in FIG. 2) in the furnace circuits. These induce a pressure drop which improves the ability of the circuits to accept heat upsets and maintain resulting flow changes within design limits. In addition, a satisfactory characteristic of flow is achieved by crossing-over the tubes within a wall from a zone of high heat absorption to a zone of low heat absorption, and vice versa. FIG. 2 illustrates the functional thinking of the cross-over arrangement. Usually, a furnace enclosure wall is divided into a plurality of panels, each panel containing a group of tubes. The panels may be about ten feet wide. The cross-over zone 94 is positioned about the burner zone 95 at a height determined by the requirements of a particular furnace design. Referring to FIG. 2, the letters (A, B, C, D) and the numbers (1, 2, 3, 4) show the extent and location of the cross-over. The numbers and letters below the oblique lines represent the positions of lower circuit panels and the numbers and letters above the lines represent the positions of the circuit panels following cross-over. By way of example, circuit panel C is moved to the left, whereas circuit panel D is moved to the right to replace the position in the furnace periphery previously occupied by circuit panel C. Similarly, circuit panels 1 and 2 exchange places (with respect to the periphery of the furnace) as do circuit panels A and B, and 3 and 4. If the location before cross-over is in a high heat absorption zone, then the location of the panel following crossover will be in a low heat absorption zone (that is, if there is an absorption upset across the furnace width). Specific details of the cross-over are illustrated in FIGS. 5, 5A and 58. Alternate levels are selected for crossing-over a circuit to permit pulling out alternate tubes from the wall at one level and the remaining alternate tubes from another level. For instance, with reference to FIG. 5, tubes C3, C5, C7, and C9 are pulled out from the circuit righthand location and reintroduced at the circuit lefthand location at the same approximate level. The remaining tubes of that circuit then are crossed-over at a second level (not shown). Present day generators are top-supported. By the above cross-over arrangement, the support load for a wall may be transmitted through the elevation where the cross-over occurs. For every tube pulled out of a wall location, a cross-over tube from another location in the furnace is inserted. The alternating pattern provides a continuous sequence of adjacent tubes which can be welded together to form an all-welded furnace enclosure for load transmittal.

The generator illustrated in FIGS. 3 and 4 is about 800 MW in size. The feed flow is transmitted from the economizer inlet header 38a through the economizer 38 to the economizer outlet header 38b. This flow then is transmitted by means of line 40 to the generator downcomer 42. In the case of an 800 MW unit, two

downcomers would be employed, one on each side of the unit, each apparoximately 16 inches in outside diameter. Each downcomer would be provided with a water jet-pump of the type described with reference to FIG. 8. When the unit is operating in the recirculating mode (jet-pump capability not yet overcome by feed pressure), fluid flows through the downcomers 42 and through the feeders 96 to panel circuit inlet headers 98 in the front, rear and furnace side walls. Flow is then upward through the lower furnace 102 to an elevation above the burner zone (determined by design calculations) where circuit cross-over is accomplished. Typialu hes zs ins spasms f91i ..90 flssasratq r 1 inch OD and l we inch centers, for the lower furnace, 1 inch OD for the crossover tubes outside the fur nace, and 1 A inch OD on 1 1% inch centers, for the upper furnace. Flow continues through the upper furnace panels to the furnace outlet headers 106, which connect to separators 50 by connecting tubes 58. For subcritical pressure operation, at less than about 50 percent load, vapor and liquid mixture enters the separators 50, vapor separating from the mixture and then routing through the riser tubes to the roof circuit and division wall tubes 16, and then through support stringer tubes 109 to the primary superheater inlet header 110, through the primary superheater 18, and then the finishing superheater 22 to the finishing superheater outlet 112. Separated drain flow from the separator 50 flows through connecting pipes 54 to the manifold collecting header 48. This flow then recirculates as previously described through the jet pump 72 and downcomers 42 to the furnace inlet headers.

It is contemplated that the large peaking vapor generator of FIGS. 3 and 4 will be gas or oil fired, and thus the furnace periphery will be of a dimension which will permit a once-up-through flow with satisfactory mass flow rates while utilizing tubes no smaller than 1 inch in outside diameter. In particular, furnace circuit mass flow rates at full load are sufficiently high (lower furnace mass flow rates are about 2.0 X 10 lb/hr-sq. ft., and upper furnace mass flow rates are about 1.0 X 10 lb/hr-sq. ft.) to insure satisfactory operation with supercritical fluid at full load. At the same time, satisfactory operation with vapor-liquid mixtures at lower loads while operating at variable pressure is achieved.

Coal fired vapor generators on the other hand require a greater furnace periphery, with less through put flow per foot of periphery, and a once-up-through circuitry with tube sizes no smaller than 1 inch OD would not meet good design practice. To utilize the jet-assist natural circulation principle for this type of unit, an additional circuit is employed in the side wall of the vapor generator. A schematic arrangement of this circuitry modification is illustrated in FIG. 9 and arrangements are shown in FIGS. 10 and 11.

With reference to the figures, the feedwater throughput is first through economizer 1 l4 and then into a first furnace pass 116 which encompasses the lower furnace side wall center portion (FIG. 10). Downcomers 118 (FIG. 10) connect the economizer and pass 116. The flow then is directed to two additional downcomers 120 (one on each side) for the generator, through connecting piping 122 and 124, the latter providing the driving flow for jet pump 126. For this unit, the minimum startup flow is approximately 15 percent of full load flow, which not only provides the driving flow for the jet pump but also insures an adequate mass flow rate in the first pass 116. Valve 128 in connecting pipe 122 functions as described above to provide the jet-pump driving force, approximately percent of full load flow being transmitted through the connecting pipe 124 to the jetpump nozzle.

From the downcomers 120, the flow is then transmitted into the second pass 130 of the generator, which comprises the front, rear and side wall end panels, the output from these panels then going to separators 132. Downcomers 133 transmit the liquid flow from the separators back to jet pumps 126 to complete the natural circulation loop. This jet assisted natural circulation in the second pass will be maintained to approximately 45 percent load, at which load, feed pressure overcomes the jet pump capability closing check valves 134. With the check valves closed, the fluid flow is shifted from recirculation to once-through flow.

From the separators 132, the vapor flow is routed to a third furnace pass 136 which constitutes the upper side wall center portions of both sides of the generator. This throughput flow then is transmitted into the partial furnace division walls 138 (pass 4), and then remaining downstream circuits of the generator.

For the generator of FIGS. 9-11, the jet pump preferably is located at the downcomer lower extremities (see FIG. 10) so that any vapor formed in the first furnace pass will not reduce the total downcomer hydrostatic head. Preferably, however, significant quantities of vapor would not be formed.

Advantages of the invention should now be apparent. In particular, the invention permits a minimum start-up flow, with a minimum external start-up system and controls required for this initial operation.

Essentially, a conventional natural circulation loop is employed. The use of a jet pump to assist circulation and a by-pass to accommodate the feed pump flow which is in excess of the vapor generated add little to the cost and complexity of the operation of the generator. The design of the furnace circuitry utilizing a natural circulation loop also permits the generator, including the furnace circuitry, to be operated at low loads and start-up at the relatively low circuit pressure of about 1000 psi. The usual bypass and pressure breakdown systems conventionally employed with oncethrough generators are not required. Accordingly, the unit at low loads operates with a minimum of complication and system maintenance and is immediately available as a spinning reserve to the power system.

In addition, the unit is capable of true variable pressure operation throughout the load range including transition from subcritical to supercritical pressure. This permits full-arc turbine admission over the load range of the generator and results in minimal changes to turbine first stage temperature, prolonging turbine life. It also permits close matching of fluid temperature to turbine metal temperature.

During operation at variable pressure with essentially a natural circulation loop at low loads and oncethrough at higher loads, functionally satisfactory circuit performance in the furnace circuitry over the load range of the generator is achieved. The transition from a recirculation type of operation to a oncethrough type of operation is inherently automatic within the circuitry, so that complex controls are not required in this respect.

In that the generator is capable of once-through operation at high loads, there is a minimum of pressure part weight which makes possible rapid response to load demand changes with minimal overfiring. Other design features of the generator coupled with minimum pressure part weight give low equipment costs, fabbrication and erection costs.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

What is claimed is:

1. A method of operating a vapor generator of the once-through type comprising the steps of establishing the circulation rate in the generator at normal loads by the generator feed pump, the evaporation rate of the generator being equal to the circulation rate;

varying the pressure within the generator in proportion to load;

carrying out the evaporation in the furnace enclosure at lower loads only to a vapor-liquid mixture;

separating the vapor and liquid portions in said mixture;

recirculating the liquid portion of the mixture to the furnace enclosure at such lower loads at a density by which natural circulation in part provides the driving force of recirculation;

maintaining at least a minimum generator feed pump flow;

mixing said feed pump flow with the recirculated flow at the lower loads in such a way as to effect a transfer of energy from the feed pump flow to the recirculated flow and thereby increase the recirculation rate at the lower loads; and

converting the generator from recirculated flow to once-through flow, and vice versa, substantially at that load and pressure at which there is essentially little or no driving force by natural circulation.

2. The method of claim 1 wherein the transition from recirculated flow to once-through flow and vice versa occurs at about 50 percent load, evaporation to dryness or to a high quality mixture occurring in the furnace enclosure at said load.

3. The method of claim 2 in which the generator pressure at full load is supercritical.

4. The method of claim 1 wherein the ratio of feed pump flow to recirculated flow is increased with increased load and pressure at said lower loads.

5. The method of claim 1 wherein the minimum driving feed pump flow utilized is approximately l0 percent of the full load feed pump flow.

6. The generator of claim 5 wherein the feed pump flow is that required to maintain a predetermined circulation rate in the furnace enclosure, further including the step of returning excess liquid flow from the furnace enclosure to constitute at least in part the feed pump flow.

7. The method of claim 4 further including the step of preheating the feed pump flow in tubes of the furnace enclosure prior to mixing the recirculated flow and feed pump flow.

8. The method of claim 4 wherein the flow in the furnace enclosure is of the once-up-through type at loads exceeding about 50 percent load.

9. The method of claim 4 wherein the feed pump flow and recirculated flow are mixed in a jet pump with a portion of the feed pump flow providing the driving force, including the step of bypassing said jet pump with the remaining portion of feed pump flow to maintain a predetermined pressure ratio of the jet pump driving flow to the driven recirculated flow.

10. A vapor generator of the once-through type including a radiantly heated furnace enclosure and feed pump means arranged to pass a liquid flow through the enclosure, the improvement comprising means for establishing essentially the same pressure throughout the generator and for varying said pressure in response to load;

separator means for receiving the flow from the generator furnace enclosure when such flow is a vaporliquid mixture;

recirculation means for recirculating the liquid portion of said flow back to the furnace enclosure, said separation means and recirculation means establishing with the furnace enclosure a natural circulation loop;

means for mixing the recirculated flow with said feed pump flow in such a way as to effect a transfer of energy from the feed pump flow to the recirculated flow and thereby increase the recirculation ratio by the feed pump flow; and

valve means for interrupting the recirculated flow permitting the generator flow to be either in part by natural circulation or totally once-through.

11. The generator of claim wherein the transition from natural to once-through flow and vice versa occurs at an intermediate load point, the generator furnace enclosure being sized to evaporate the flow to a high quality mixture or to dryness at said load point.

12. The generator of claim 10 including at least one downcomer, the recirculated flow from said separator means being into said downcomer.

13. The generator of claim 12 in which the means for mixing the recirculated flow with the feed pump flow is a jet pump comprising a venturi section interposed in said downcomer, a nozzle for introducing a jet stream into said venturi section, and means connecting said nozzle with said feed pump.

14. The generator of claim 13 further including conduit means for receiving a portion of the flow from said feed pump means and for mixing said portion with the recirculated flow at a point downstream of the jet pump, said conduit means including valve means arranged to maintain a predetermined pressure ratio between the pressure in said nozzle and the recirculated flow in said downcomer upstream of the venturi section.

15. The vapor generator of claim 10 including header means for receiving the liquid flow from said separator means, flow detection means for detecting the rate of flow from said feed pump means, and bypass means for bypassing at least part of the flow from said header means to a point upstream of said flow detection means to prevent flooding of said separator means.

16. The generator of claim 15 wherein said bypass means includes a valve responsive to liquid level in said separator means.

17. The generator of claim 16 wherein the bypass flow is routed to the generator deaerator or high pressure heaters for recovery of heat therein.

18. The generator of claim 10 wherein said furnace enclosure comprises a circuitry which is of the onceup-through type.

19. The generator of claim 18 in which said furnace enclosure is oil or gas fired.

20. The generator of claim 18 wherein said furnace enclosure comprises a plurality of upright panels, each panel comprising a plurality of parallel upright tubes, the enclosure including cross-over means intermediate the upper and lower elevations of the enclosure by which the flow in panels in a high heat absorption area of the enclosure is transferred to an area of low heat absorption, and vice versa.

21. The generator of claim 20 wherein the cross-over means is positioned above the enclosure burner zone.

22. The generator of claim 20 further including a plurality of resistances interposed at the entrance to the furnace enclosure circuits for equalizing the flow in the circuits.

23. The generator of claim 10 including a plurality of separator means, each separator means comprising an outer cylindrical upright vessel, an inner conduit extending axially within the vessel including a closed upper end, a plurality of curved arms extending radially from said conduit upper end for transmitting the flow in said conduit tangentially along the inner wall of the vessel, the conduit being arranged to receive the flow from the furnace enclosure.

24. The generator of claim 10 in which said generator ic coal fired, further including means for transmitting the feed pump flow first to at least one flow circuit constituting a portion of the furnace enclosure and then mixing said feed pump flow with recirculated flow, the combined recirculated and feed pump flow then being transmitted through remaining circuits of the furnace enclosure.

25. The generator of claim 24 in which said at least one flow circuit constitutes panels in the center portions of the lower side walls of the furnace enclosure.

26. A method of operating a vapor generator of the once-through type at low loads and start-up comprising the steps of:

carrying out evaporation in the furnace enclosure only to a vapor-liquid mixture;

separating the vapor and liquid portions of the mixture;

recirculating the liquid portion of the mixture to the furnace enclosure at a density by which natural circulation in part provides the driving force for the recirculation;

maintaining at least a minimum generator feed pump flow; and

mixing said feed pump flow and recirculated flow so that the feed pump flow provides the remainder of the driving force for recirculation.

27. The method of claim 26 wherein the ratio of feed pump flow to recirculated flow is increased with increased load and pressure in the generator.

28. The method of claim 27 wherein the flow in the generator is converted from recirculated flow to oncethrough flow, and vice versa, substantially at that load and pressure at which there is essentially little or no driving force by natural circulation. 

1. A method of operating a vapor generator of the once-through type comprising the steps of establishing the circulation rate in the generator at normal loads by the generator feed pump, the evaporation rate of the generator being equal to the circulation rate; varying the pressure within the generator in proportion to load; carrying out the evaporation in the furnace enclosure at lower loads only to a vapor-liquid mixture; separating the vapor and liquid portions in said mixture; recirculating the liquid portion of the mixture to the furnace enclosure at such lower loads at a density by which natural circulation in part provides the driving force of recirculation; maintaining at least a minimum generator feed pump flow; mixing said feed pump flow with the recirculated flow at the lower loads in such a way as to effect a transfer of energy from the feed pump flow to the recirculated flow and thereby increase the recirculation rate at the lower loads; and converting the generator from recirculated flow to once-through flow, and vice versa, substantially at that load and pressure at which there is essentially little or no driving force by natural circulation.
 2. The method of claim 1 wherein the transition from recirculated flow to once-through flow and vice versa occurs at about 50 percent load, evaporation to dryness or to a high quality mixture occurring in the furnace enclosure at said load.
 3. The method of claim 2 in which the generator pressure at full load is supercritical.
 4. The method of claim 1 wherein the ratio of feed pump flow to recirculated flow is increased with increased load and pressure at said lower loads.
 5. The method of claim 1 wherein the minimum driving feed pump flow utilized is approximately 10 percent of the full load feed pump flow.
 6. The generator of claim 5 wherein the feed pump flow is that required to maintain a predetermined circulation rate in the furnace enclosure, further including the step of returning excess liquid flow from the furnace enclosure to constitute at least in part the feed pump flow.
 7. The method of claim 4 further including the step of preheating the feed pump flow in tubes of the furnace enclosure prior to mixing the recirculated flow and feed pump flow.
 8. The method of claim 4 wherein the flow in the furnace enclosure is of the once-up-through type at loads exceeding about 50 percent load.
 9. The method of claim 4 wherein the feed pump flow and recirculated flow are mixed in a jet pump with a portion of the feed pump flow providing the driving force, including the step of bypassing said jet pump with the remaining portion of feed pump flow to maintain a predetermined pressure ratio of the jet Pump driving flow to the driven recirculated flow.
 10. A vapor generator of the once-through type including a radiantly heated furnace enclosure and feed pump means arranged to pass a liquid flow through the enclosure, the improvement comprising means for establishing essentially the same pressure throughout the generator and for varying said pressure in response to load; separator means for receiving the flow from the generator furnace enclosure when such flow is a vapor-liquid mixture; recirculation means for recirculating the liquid portion of said flow back to the furnace enclosure, said separation means and recirculation means establishing with the furnace enclosure a natural circulation loop; means for mixing the recirculated flow with said feed pump flow in such a way as to effect a transfer of energy from the feed pump flow to the recirculated flow and thereby increase the recirculation ratio by the feed pump flow; and valve means for interrupting the recirculated flow permitting the generator flow to be either in part by natural circulation or totally once-through.
 11. The generator of claim 10 wherein the transition from natural to once-through flow and vice versa occurs at an intermediate load point, the generator furnace enclosure being sized to evaporate the flow to a high quality mixture or to dryness at said load point.
 12. The generator of claim 10 including at least one downcomer, the recirculated flow from said separator means being into said downcomer.
 13. The generator of claim 12 in which the means for mixing the recirculated flow with the feed pump flow is a jet pump comprising a venturi section interposed in said downcomer, a nozzle for introducing a jet stream into said venturi section, and means connecting said nozzle with said feed pump.
 14. The generator of claim 13 further including conduit means for receiving a portion of the flow from said feed pump means and for mixing said portion with the recirculated flow at a point downstream of the jet pump, said conduit means including valve means arranged to maintain a predetermined pressure ratio between the pressure in said nozzle and the recirculated flow in said downcomer upstream of the venturi section.
 15. The vapor generator of claim 10 including header means for receiving the liquid flow from said separator means, flow detection means for detecting the rate of flow from said feed pump means, and bypass means for bypassing at least part of the flow from said header means to a point upstream of said flow detection means to prevent flooding of said separator means.
 16. The generator of claim 15 wherein said bypass means includes a valve responsive to liquid level in said separator means.
 17. The generator of claim 16 wherein the bypass flow is routed to the generator deaerator or high pressure heaters for recovery of heat therein.
 18. The generator of claim 10 wherein said furnace enclosure comprises a circuitry which is of the once-up-through type.
 19. The generator of claim 18 in which said furnace enclosure is oil or gas fired.
 20. The generator of claim 18 wherein said furnace enclosure comprises a plurality of upright panels, each panel comprising a plurality of parallel upright tubes, the enclosure including cross-over means intermediate the upper and lower elevations of the enclosure by which the flow in panels in a high heat absorption area of the enclosure is transferred to an area of low heat absorption, and vice versa.
 21. The generator of claim 20 wherein the cross-over means is positioned above the enclosure burner zone.
 22. The generator of claim 20 further including a plurality of resistances interposed at the entrance to the furnace enclosure circuits for equalizing the flow in the circuits.
 23. The generator of claim 10 including a plurality of separator means, each separator means comprising an outer cylindrical upright vessel, an inner conduit extending axially within the vessel including a closed upper end, a plurality oF curved arms extending radially from said conduit upper end for transmitting the flow in said conduit tangentially along the inner wall of the vessel, the conduit being arranged to receive the flow from the furnace enclosure.
 24. The generator of claim 10 in which said generator ic coal fired, further including means for transmitting the feed pump flow first to at least one flow circuit constituting a portion of the furnace enclosure and then mixing said feed pump flow with recirculated flow, the combined recirculated and feed pump flow then being transmitted through remaining circuits of the furnace enclosure.
 25. The generator of claim 24 in which said at least one flow circuit constitutes panels in the center portions of the lower side walls of the furnace enclosure.
 26. A method of operating a vapor generator of the once-through type at low loads and start-up comprising the steps of: carrying out evaporation in the furnace enclosure only to a vapor-liquid mixture; separating the vapor and liquid portions of the mixture; recirculating the liquid portion of the mixture to the furnace enclosure at a density by which natural circulation in part provides the driving force for the recirculation; maintaining at least a minimum generator feed pump flow; and mixing said feed pump flow and recirculated flow so that the feed pump flow provides the remainder of the driving force for recirculation.
 27. The method of claim 26 wherein the ratio of feed pump flow to recirculated flow is increased with increased load and pressure in the generator.
 28. The method of claim 27 wherein the flow in the generator is converted from recirculated flow to once-through flow, and vice versa, substantially at that load and pressure at which there is essentially little or no driving force by natural circulation. 